Add tests

[juanitorduz]

Add test of some missing functions.

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📚 Documentation preview 📚: https://pymc-marketing--1391.org.readthedocs.build/en/1391/

[codecov]

Codecov Report

Attention: Patch coverage is 91.66667% with 3 lines in your changes missing coverage. Please review.

Project coverage is 94.59%. Comparing base (7a41a72) to head (1531e86).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
pymc_marketing/mmm/base.py	91.42%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1391      +/-   ##
==========================================
+ Coverage   93.80%   94.59%   +0.78%     
==========================================
  Files          48       48              
  Lines        5198     5199       +1     
==========================================
+ Hits         4876     4918      +42     
+ Misses        322      281      -41     

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